Knurlizing Pistons?

[NE57]

I ran across a reference to knurlizing pistons for a tighter fit in the bore. Is this reliable? How much expansion of the piston is practical? Would you still measure piston clearance in bore with a mic/snap gauge or feeler gauge?

When I read about it the thought occurred to me that if my cylinders clean up with just a good honing I might be able to reuse my Jahn's forgings if knurling was a viable option as opposed to boring plus a new set of pistons. I don't know how deep the rust goes but might be worth a shot.

Thoughts? Recommendations?

[speedicusmaximus]

If you mean Knurling, as in creating that criss-cross, diamond-like raised surface finish on the piston skirt, I wouldn't touch it ! There's no control over the tolerances on the knurling - how much the material rises under pressure from the knurling tool. Get new pistons !

Mike

[oldngood]

I ran across a reference to knurlizing pistons for a tighter fit in the bore. Is this reliable? How much expansion of the piston is practical? Would you still measure piston clearance in bore with a mic/snap gauge or feeler gauge?

When I read about it the thought occurred to me that if my cylinders clean up with just a good honing I might be able to reuse my Jahn's forgings if knurling was a viable option as opposed to boring plus a new set of pistons. I don't know how deep the rust goes but might be worth a shot.

Thoughts? Recommendations?

I tried knurling back in the early 1980's when "low bucking" engine rebuilds, it will work but the knurl quickly wears off, and in a short time you have a piston slap noise in the engine, like a dull "tonk-tonk-tonk" sound at idle

if the piston/bore specs is too high, get new slugs and the rebuild will last for years, instead of for months

[NE57]

Thanks. I also read something about using teflon buttons for the same thing. If that's not a good idea either i guess I wll sell the Jahns and buy new slugs.

[mart]

Thanks. I also read something about using teflon buttons for the same thing. If that's not a good idea either i guess I wll sell the Jahns and buy new slugs.

-----------------------------------
I agree that you don't want to knurl the pistons. If the block
needs to be bored or honed too much to get bores to clean
up, you need to get new pistons. But before you toss the
Jahns though, depending on how bad the bores are rusted
or worn, you might try honing the bores the bare minimum
you can get away with to clean them up. Remember too that
any scores or pitting below where the rings actually run in
the bore don't matter. Once you have the bores cleaned up,
do a trial fit and check the piston-to wall-clearance with the
Jahns pistons. Mic the piston skirts individually too and fit
them to each bore to get the best fit. The Jahns are probably
forged and probably fairly old too. The older forgings,
particularly with the alloys used back in the '50's and 60's,
needed to run more piston-to-wall than most modern style
forgings. I don't know what clearance your particular Jahns
pistons call for but if they're forged and of 1950's, '60s or
even early '70's vintage, the reccommended clearance is
likely to be around .008 or 009 piston-to-wall. If that's
the case, 'you *might* get lucky and have the bores clean
up and still be within spec. By the way, if you think that
running .008 to .009 piston-to-wall might give you a fair
bit of piston slap, you're right! Back in the old days, you
could always tell when somebody was running forged slugs
by the steady "tick-tick-tick" sound until the engine
warmed up!

mart
=============================================

[oldngood]

Thanks. I also read something about using teflon buttons for the same thing. If that's not a good idea either i guess I wll sell the Jahns and buy new slugs.

well teflon buttons are used on top fuel/funny car pistons, but really aren't a street deal- by the time you machine the holes, but the buttons, and put them in the pistons, you could just buy 8 new pistons and have a much better job

you're now getting down to the reason I sold my 331's, if they need pistons and a rebore, the rebuild gets mighty expensive quick, and you have to ask yourself is it worth it

of course, running open headers or header mufflers will drown out piston slap ...(chuckle...)

[NE57]

I hafta admit, cost is a factor in exploring reusing my pistons. When I first looked at the motor to buy it I automatically figured on a rebore. But the more this recession goes on the farther away hemi parts become. I have my own business and its gotten pretty crappy in the 6 months since I bought the motor.

How do you spell Hemi Frustration? ...B*U*S*I*N*E*S*S**S*U*C*K*S

I have to be careful not to make a stupid mistake out of frutration and the urge to just get on with it.

A really stupid mistake would be, "Screw it, I'm just gonna plop a smallblock in it".

>Mic the piston skirts individually too and fit
them to each bore to get the best fit.<

Yeah I ordered a 0-6 inch micrometer set and a set of snap gauges, figuring I would go ahead and see what honing does. The worst that can happen is i find out I need new pistons and a bore. Earth shattering news. I can't really screw up the cylinders if I try this at home, can I? With one of those $30 spring loaded hones from NAPA?

While we're on about honing rusty scored cylinders..what would be the criteria for judging whether they cleaned up enough? Obviously, remaining heavy pitting is no good but how about fine pitting? Is there a point beyond which its OK?

In my own business "good enough' is usually NOT good enough. How picky should I be about bore finish?

[David Hunt]

Good day, I hear what you are saying as I too am facing this hurdle. What I do is use an etching ACID wheel cleaner by a company I wont name such as EAGLE ONE. But spar it in the cylinders after pistons have been removed. Only worry about how clean the top 1/2 of cylinder bore is condition after wiping out the acid. The inside will clean up as if new. Shine a light bulb on end of cord into the cylinder that you are checking. Think about an over size ring assembly. If not good go sleving of all 8 cylinders as to return to stock size letting you reuse ROUND stock pistons you removed in the first place. This is what I do with a medium fine hone finish CROSS-Hatched pattern. Have a Mopar Day Inspector Mopar

[NE57]

I can understand how acid would remove the rust itself, but won't there still be pitting? Its the pits and scores that are really at the heart of the matter.

[Paul]

Even if you bore & finish hone your cylinders , there's still a chance that a few pits or imperfections are still visable. If it's a couple of minor blemishes, the chances are it's not enough surface area to make a difference.
Note: on Jahns & various other forged pistons, you may be surprised how much more piston clearance is acceptable over cast. I believe a earlier poster already handed you those clearances. There's a few companies who did knurl their skirts, I believe Anson was one of them. You won't be able to pull this off your self. They had nice neet uniform patterns. I personally try to find machinists w/ torque plates, even though these are excellenet high nickel castings. If you have doubts about your set ups, wait till you are financially sound. These Hemis really last when built right. Slight mods yeild excellent hp & torque over most motors. They rated hp on these to the ground. The intake ports were tiny even on 4bl applications. Most of the exhaust manifolds offered very limited flow. Hp ratings on these are almost unfair/in comparison to modern standards. If you plop a 350 Chevy in for convenience, good luck with your resale value.

[johnny5]

The last time I had a piston knurled was 20 years ago and it worked great (in my lawnmower)